(1) Field of the Invention
This invention relates to systems for aligning two or more parallel pipes, cables or other elongate elements during offshore operations, for example in a ‘piggyback’ arrangement during pipelaying.
It is often desirable to install two or more elongate elements along the same subsea route, such as a primary larger-diameter pipe for carrying hydrocarbons and a secondary smaller-diameter pipe for carrying water, gas or chemicals used to produce hydrocarbons.
Whilst pipes will be used as an example in this specification, an element need not be a pipe for carrying fluids but could instead be a cable for carrying power or data. A secondary element will usually be of much smaller diameter (typically <20 cm) than a primary element, but a difference in size between the elements is not essential to the invention in a broad sense.
Where elements such as pipes or cables are to follow the same route, it may be beneficial to install the elements simultaneously. This is commonly achieved by a piggyback technique where one or more secondary elements are attached by a succession of clamps to a primary element on a pipelay vessel, and the elements are then launched together in parallel toward the seabed.
Installation of a piggyback pipeline usually involves unspooling the secondary pipe on a pipelay vessel. The primary pipe may also be unspooled in a reel-lay arrangement although it could be fabricated on the pipelay vessel, for example in an S-lay operation.
(2) Description of Related Art
A typical reel-lay vessel 10 shown schematically in FIG. 1 is fitted with a storage and deployment reel 12 for deploying a primary pipe 14 and has an adjustable lay ramp 16 that is capable of deploying a range of products at varying lay angles, which may be from circa 20° to 90° to the horizontal. The inclination of the lay ramp 16 is determined by the depth of water in which the pipeline is being laid and by the characteristics of the pipeline, such as its diameter and stiffness.
In downstream succession from the reel 12, the lay ramp 16 carries a guide chute 18 for guiding the primary pipe 14; a pipe straightener 20 for straightening the primary pipe 14; a track-type tensioner 22 for gripping the primary pipe 14 between articulated tracks; and a hold-off clamp 24 for clamping the primary pipe 14 whenever the tensioner 22 releases the primary pipe 14. A travelling clamp could be used instead of a track-type tensioner 22; references in this specification to a tensioner should be taken to include a travelling clamp unless the context demands otherwise.
As FIG. 2 shows, a piggyback reel 26 can be fitted to a vessel 10 for deploying a secondary element such as a secondary pipe 28 with the primary pipe 14 when operating in piggyback mode. In that mode, a piggyback chute 30 guides the secondary pipe 28 and the secondary pipe 28 is brought into alignment with the primary pipe 14, such that the secondary pipe 28 lies parallel to the primary pipe 14 downstream of the tensioner 22. The secondary pipe 28 then lies directly above the longitudinal centreline of the primary pipe 14 or, when the primary pipe 14 is vertical, directly aft of the longitudinal centreline of the primary pipe 14. The secondary pipe 28 is then ready to be clamped to the primary pipe 14 at work platforms in a shelter 32 on the lay ramp 16 between the tensioner 22 and the hold-off clamp 24.
In practice an additional straightener may be used for the secondary pipe 28 downstream of the piggyback chute 30 but this has been omitted from FIG. 2 for clarity. Also, the secondary pipe 28 may go through an additional tensioner but such a tensioner may not be required and has also been omitted for clarity.
In a prior art piggybacking arrangement, it is known for a secondary pipe 28 to be diverted entirely around the tensioner 22 before being aligned with the primary pipe. This makes it difficult to align the secondary pipe 28 without overbending it or requiring additional straightening, unless there is a substantial and disadvantageous gap under the tensioner 22. The heavy tensioner 22 should be mounted as low as possible on the lay ramp 16 to aid the stability of the vessel 10.
U.S. Pat. No. 5,975,802 to Willis (Assignee: Stolt Comex Seaway Ltd.) discloses a known piggyback arrangement in detail, including the relationship between the paths of a primary pipe and a secondary pipe as they pass over their respective chutes and are brought together for clamping. In the example shown in U.S. Pat. No. 5,975,802, the primary pipe is fabricated on board the pipelay vessel and the secondary pipe is unspooled from a reel, although it will be clear to the skilled reader that both pipes could be spooled with the addition of a storage and deployment reel for the primary pipe, as in FIG. 2. The content of U.S. Pat. No. 5,975,802 is incorporated herein by reference, as technical background to the present invention.